Microwave heating apparatus



. Filed June 15, 1967 Sheet April 1969- TORAO NAGAI ETAL 3,437,777

MICROWAVE HEATING APPARATUS T0240 /VA6A/, flnsauoaa ,4/MEA, S/Yua-oHAM/(A3 INVENTORS p i 1969. TORAO NAGAI ETAL 3,

MICROWAVE HEATING APPARATUS Filed Ju e 15, 1967 Sheet 2 of 4 4 0 FIG.if; Z, -41b x 6 2 b b i ao 44 m L713 51 T FIG. 86%;.

16 52- Q -27a 45 31 m A ril 8, 1969 Filed June 13, 1967 OUTPUT POWERTORAO NAGAI ETAL MICROWAVE HEATING APPARATUS Sheet 4 of4 do -60 ab :60I210 440 DISTANCE [x (mm) United States Patent 3,437,777 MICROWAVEHEATING APPARATUS Torao Nagai, Yokohama-shi, Masanobu Aiura and ShuzoTanaka, Tokyo, and Shu Chiba, Yokohama-shi, Japan, assignors to TokyoShibaura Electric Co., Ltd., Kawasaki-shi, Japan, a corporation of JapanFiled June 13, 1967, Ser. No. 645,775 Claims priority, applicationJapan, June 17, 1966, ll/38,890; June 30, 1966, 41/42,225; Oct. 15,1966, 41/167,423; Nov. 19, 1966, ll/75,756

Int. Cl. H05b 9/06 US. Cl. 21910.55 13 Claims ABSTRACT OF THE DISCLOSUREA Waveguide connection interconnected between a microwave oscillator forsupplying microwave energy and an enclosed heating cavity defined by ametal wall structure comprises at least one circulator in the form of abranched rectangular waveguide arranged to lead microwave energyreflected from the heating cavity to a microwave absorber so as toprevent the reflected microwave from returning to the microwaveoscillator.

Background of the invention This invention relates to microwave heatingapparatus and more particularly to such apparatus provided with animproved arrangement for protecting the microwave oscillator includedtherein during no-load and light load operating conditions of theapparatus.

As is well known in the art a microwave heating apparatus is utilized toirradiate an object to be heated with microwave energy having afrequency of about 2450 mc./sec., for example, whereby to heat theobject uniformly. Although such apparatus have been mainly used forheating foods or freezed foods and are called as an electronic cookingapparatus, recently applications thereof to heating, molding and workingof synthetic resins and to heating and drying of pottery have becomecommon.

Generally, a microwave heating apparatus comprises a box-like heatingcavity or chamber defined by metal walls, a microwave oscillator,usually employing a magnetron, and a waveguide connection coupling theoscillator to the cavity, for supplying microwave energy generated bysaid microwave generator into the cavity. An object or material to beheated, for example, a food-stuff is inserted in the cavity anddielectrically heated by the irradiation of microwave. However, underlight load or no-load condition wherein the object inserted in thecavity is small or there is no object, the most part of the microwaveenergy supplied to the cavity will be reflected to flow in the oppositedirection back to said magnetron oscillator through the waveguideconnection. This results in breaking down of the microwave oscillator ormagnetron. In such a case, especially the cathode electrode of themagnetron often is damaged by back bombardment, thus the backbombardment not only shortens the life of expensive magnetron but alsoultimately destroys it.

As an approach for preventing troubles caused by re flected waves, ithas been proposed to connect a Faraday rotation type or resonance typeisolator in said waveguide connector for the purpose of isolating theoscillator from the cavity and to absorb and attenuate microwave energyreflected from the cavity as described in U.S. patents, Nos. 2,776,412,2,929,905 and 3,210,513. However, when using a Faraday rotation typeisolator, a Faraday rotator is provided in a circular waveguide topolarize electromagnetic wave by 45 thus imparting thereto anon-reciprocal property. This arrangement required rectangularwaveguides to be connected to the input, output and the third branch.The diameter of the circular waveguide required for the transmission ofa microwave having a frequency of the order of 2450 mc./ sec. is morethan 10 centimeters. In addition, in order to connect rectangularwaveguides to the both ends of a circular waveguide as well as the thirdbranch, it is necessary to use rectangular-circular waveguide adapter,thus greatly complicating the construction of the waveguide arrangement.Moreover, it is necessary to rotate the plane of electric field by 45 byapplying from an external magnetic field to a ferrite element providedin said circular waveguide.

As is well known in the art, the characteristics of ferrite element arecaused to vary by temperature variation, which results in the variationof the angle of rotation of the plane of the microwave, thusdeteriorating the characteristics of the isolator. This will causeattenuation of the useful microwave transmitted to the cavity. Further,as it is necessary to mount the ferrite element at the center of thewaveguide there is a problem in mounting it. In microwave communicationwherein the average power of the electric wave transmitted is usuallyless than few watts there is no serious problem. However, in microwaveheating apparatus wherein the average power of microwave transmittedexceeds several hundred watts, there is a tendency that the ferriteelement is overheated.

On the other hand in the apparatus utilizing a resonance type isolatorusing the phenomenon of ferromagnetic resonance, a thin ferrite slab isutilized. Thus, the ferrite element does not act as an isolator withoutbeing excited by an external cross-magnetic field of more than 2000 to3000 gausses thus requiring extremely strong and bulky excitingapparatus. Further as the microwave reflected from the cavity isabsorbed and attenuated by the ferrite slab, its temperature increases.By this reason it is necessary to cool the ferrite slab by means of alarge cooling device.

In each of the above described prior apparatus for preventing troublescaused by reflected wave in microwave heating apparatus the constructionof the isolator was complicated and bulky so that many troubles wereencounted in the practical use of the microwave heating apparatus.Especially, in the heating apparatus handling average power of microwaveof more than 1 kw., the isolator having a construction as abovedescribed is not only uneconomical but it also can be difficult toprovide satisfactory characteristic.

Summary of the invention.

One of the objects of this invention is to provide an improved microwaveheating apparatus including a novel circulator between a cavity of theheating device and a microwave oscillator which is carefully designed todecrease heating eflect and to prevent microwave reflected from thecavity from reaching the microwave oscillator under light load andno-load conditions effectively.

According to one embodiment of this invention there is provided amicrowave heating apparatus comprising a microwave oscillator forsupplying microwave energy, a metal wall structure defining asubstantially closed heating cavity, and a waveguide connector betweensaid oscillator and said cavity, said connector comprising a branchedrectangular waveguide including a central waveguide section, a firstrectangular waveguide branch coupled to said oscillator to receivemicrowave therefrom,

a second rectangular waveguide branch coupled to said cavity and a thirdrectangular waveguide branch, said first, second and third branchesextending radially in different directions from said central section, aferrite element mounted in said central waveguide section at acircularly polarized position, means including an exciting elementmounted on said central waveguide section against said ferrite elementto apply thereto magnetic field whereby to introduce said microwave intosaid heating cavity through said central waveguide section and saidfirst and second branches and to direct microwave reflected from saidcavity into said third branch, and a microwave absorber contained insaid third branch to absorb reflected microwave.

According to another embodiment of this invention there is provided amicrowave heating apparatus comprising a microwave magnetron oscillatorfor supplying microwave energy, a metal wall structure defining asubstantially closed heating cavity, one of the metal side Wallsdefining said cavity being substantially fiat and being provided with aninlet opening for admitting said microwave, and a waveguide connectorbetween said oscillator and said heating cavity, said connectorincluding an H plane branched T-shape waveguide comprising a centralwaveguide section, a first rectangular waveguide branch on said onemetal side wall of the cavity, said first branch being coupled to saidoscillator at one end thereof opposite to its other end joined to saidcentral waveguide section to receive microwave from said oscillator, a'second rectangular waveguide branch colinear with said first branch,being coupled to said microwave inlet opening at one end thereofopposite to its joint to said central section, and a third rectangularwaveguide section perpendicular to said first branch, said first,second, and third branches being branched from said central waveguidesection in the H plane parallel to said side wall in three directions toform T-shaped configuration, at least one circular disc shaped ferriteelement mounted on the inner surface of opposing broad side walls ofsaid central waveguide section at a position to cause said microwavetransmitted through said central waveguide section to create asubstantially circularly polarized magnetic field, said ferrite elementhaving a fiat outer surface contacting said inner surface, meansincluding an exciting element in the form of a disc shaped permanentmagnet which is mounted on the outer surface of one of said broad sidewalls to oppose said ferrite element to apply thereto a magnetic fieldacross said central waveguide section whereby to transmit the microwavein the forward direction into said heating cavity through said centralwaveguide section and through said first and second branches and todirect microwave reflected from said heating cavity toward said thirdbranch and an electric wave absorber provided in said third branch.

This invention will be more fully set forth in the following detaileddescription with reference to the accompanying drawings, and thefeatures of novelty which characterize this invention will be pointedout with particularity in the claims annexed to and forming a part ofthis specification.

Brief description of the drawings FIG. 1 is a schematic perspective viewof one embodiment of the microwave heating apparatus according to thisinvention;

FIG. 2 shows a cross section of a rectangular waveguide utilized intheapparatus shown in FIG. 1;

FIG. 3 is a cross section view taken along a line IIIIII in HQ. 1;

FIG. 4 is a diagrammatic representation of the principle of the heatingapparatus shown in FIG. 1;

FIG. 5 is a partial view, partly broken away, showing a circulatorutilized in a. modified microwave heating apparatus;

FIG. 6 is a sectional view taken along a line VIVI in FIG. 5;

FIG. 7 is a plan view of the portion shown in FIG. 6;

FIG. 8 is a sectional view particularly illustrating the centralrectangular waveguide section employed in another modification of themicrowave heating apparatus embodying this invention;

FIG. 9 is a schematic perspective view of a still further modificationof the microwave heating apparatus of this invention;

FIG. 10 is a sectional view taken along a line XX in FIG. 9;

FIG. 11 is a schematic perspective view of yet another modification ofthe heating apparatus embodying this invention;

FIG. 12 is a longitudinal sectional view of the circulator connectoremployed in the microwave heating apparatus shown in FIG. 11;

FIG. 13A is a cross sectional view of the junction for coupling amagnetron oscillator;

FIG. 13B is a cross sectional view of the other junction; and

FIG. 14 is a graph to show the experimental result of the microwaveheating apparatus shown in FIG. '11.

Detailed description of the invention The microwave heating apparatusembodying this invention is characterized in that it utilizes a branchedtype circulator in a rectangular waveguide for eliminating troublescaused by reflected microwaves. Referring to FIG. 1 which shows aperspective view of a preferred embodiment of this invention a heatingcavity or chamber 10 is a metal box comprised by flat metal wall 11, oneof which 111 being pivotally connected at its lower edge to form anopening 12 through which an object to be heated is inserted into andremoved from the cavity. As shown in FIG. 4, a magnetron oscillator 13is provided near the heating cavity 10. The magnetron oscillator 13 issupplied with a high voltage from a source of supply, not shown, togenerate microwave energy of the frequency of about 2450 rnc./sec., forexample. The magnetron oscillator 13 and the heating cavity 10 areinterconnected by a Waveguide connection 14 including a circulator ofthe rectangular waveguide branched type. The connection 14 including thecirculator is utilized for the purpose of transmitting microwave energyfrom the magnetron oscillator 13 to the heating cavity '10 with smallloss and at a high efliciency and to attenuate microwave energyreflected from the heating cavity 10 back to the magnetron oscillator13.

The construction of the connection 14 is as follows: On one side wall ofthe heating cavity 10, for example, the upper wall 112, is mounted an Hplane branched T-shaped rectangular waveguide 15 of H mode. The H planebranched T-type rectangular waveguide 15 includes a first rectangularwaveguide branch 171, a second rectangular waveguide branch 172, and athird rectangular waveguide branch 173 which are radially branched froma control waveguide section 16 having a rectangular cross section, moreparticularly branched in three directions at the H plane, in a letter Tconfiguration. As shown in FIG. 2, each of the first, second and thirdrectangular waveguide branches 171, 172 and 173 have broad side walls18a and 18b each having a width of 76 mm. and narrow side walls 19a and1912 each having a width of 55.8 mm., for example. The cross section 20of these branches is rectangular. These first, second and thirdwaveguide branches 171, 172 and 173 are interconnected by the centralwaveguide section 16. As shown in 'FIG. '1, the first and second'bl'anehes 171 and 172 are arranged on the same straight line, and thethird branch 173 is arranged perpendicularly to the ifirst and secondbranches 171 nad 172. [he first branch 1-71 is coupled to the oscillator13 on the side opposite to the junction 161 between the first branch andthe central waveguide section 16. An antenna 22 covered by a protectivecover transparent to microwave and adapted to radiate microwave from theoscillator 13 extends through the lower broad side wall 181 of the firstbranch 171 to project therein and coupled therewith. On the other hand,the second waveguide branch 172 is coupled to the wave inlet opening 24of the heating cavity on the side of the branched portion 162 betweenthe second branch and the central waveguide section 16, and the secondbranch 172 is formed with a corner bend 25 at the portion thereofcoupled to the wave inlet opening 24. One end of the third branch 173 iscoupled to the central waveguide section 16 at a branched portion 163and the opposite end is closed. As shown in FIG. *3, a circular columnshaped ferrite element 26 is mounted in the central waveguide section'16, the .both ends of the ferrite element 26 being in contact with theopposing inner surfaces 27a and 27b of broad side walls. The position ofthe ferrite element 26 is selected so as to provide satisfactory threepart circulator action by the described branched waveguide structure.

Exciting elements 29a and 2917 are mounted to face the both ends of theferrite element 26 on the broad outer side walls 28a and 28b of thecentral waveguide section 16 to provide for the ferrite element a crossmagnetic field of the intensity of about 300 to 600 gausses in thedirection shown by arrow 30 perpendicular to the waveguide section 16.Whe excited by this crossmagnetic field the ferrite element 26 permitstransmission of the microwave supplied by the oscillator '13. Theexciting elements 29a and 29b may be electromagnets or permanent magnetsso long as they can provide aforementioned cross-magnetic field. In theillustrated embodiment a pair of disc shaped permanent magnets ofbarium-ferrite are utilized.

When the waveguide section 16, ferrite element 26 and exciting elements29a and 29b are arranged in the relation mentioned above the connection14 acts as a circulator as is well understood by those skilled in theart. Microwave energy introduced into the first branch 171 from themagnetron oscillator '13 is transmitted through the central waveguidesection 16 and appears the second branch 172, energy reflected from thecavity appears to the third branch 173 and energy input to the thirdbranch by some reasons put out to the first branch 171. FIG. 4 shows theprinciple of the operation of the circulator.

A block shaped electromagnetic wave absorber 31 is disposed in the thirdrectangular waveguide branch 173. The wave absorber 31 is made ofasintered body of clay of comb structure and Carborundum. One end of themicrowave absorber 31 is tapered to perfectly absorb microwave withoutany reflection. Then during forward transmission, the microwave from theoscillator 13 is transmitted into the cavity '10 with little lossthrough the first and second branches 171 and 172, and the wicrowavereflected from the cavity 10 is transmitted back to the second branch172, and is then absorbed and attenuated by the electromagnetic waveabsorber '31 contained in the third branch 173. In other words, althoughthe microwave generated by the oscillator 13 is transmitted to cavity 10with little loss, the microwave reflected from the cavity 10 is absorbedby the absorber 3'1 and can not flow back the magnetron so that theconnection 14 as a whole of the circulator construction operates as ifit were an isolator.

The isolating characteristic of the connection 14 exhibiting thecharacteristic of a circulator at a microwave frequency of 2450:50mc./sec. was measured over the range of temperature 0 to 100 C. Theinsertion loss in the transmission direction of microwave energy fromoscillator 13 to cavity 10 was about 0.10.2 db whereas attenuationquantity in the opposite direction, that is the attenuation quantity ofthe reflected wave was more than 25 db.

The operation of the microwave heating apparatus constructed as abovedescribed is as follows: After inserting an object to be heated 32 whichis supported by a vessel in the heating cavity 10, the front door'.111is closed. Then a prescribed operating power is supplied to themagnetron oscillator 13 to initiate its oscillation thereby to radiatemicrowave of an average power of 500 w. from the antenna 22. Thismicrowave travels straightly with little loss and without anyappreciable attenuation through the first and second branches 171 and172 to be introduced in the cavity 10.

When the user accidentally closes the door 111 without inserting theobject, or the volume thereof is small, noload or light load conditionwill be resulted, thus creating an undesirable standing wave.

As a result, a portion of the microwave energy supplied will be returnedto the connection 14 as the reflected wave. However, as has been pointedabove, since a threebranched circulator is formed by the ferrite element26 contained in the central waveguide section 16 almost all portion ofthe reflected microwave energy will be directed to the third branch 13and attenuated by the electric wave absorber 31 contained therein. Inthis manner, arrival of the reflected microwave energy at the magnetronoscillator 13 can be positively prevented.

By the use of a circulator of the waveguide branch type, the microwaveheating apparatus of this invention is advantageous over prior apparatusin the following points.

Firstly, the branched rectangular waveguide 15 comprising the main bodyof the circulator structure is simple in construction, thus enabling toconstruct it small size. In addition, the ferrite element 26 is thin andis mounted in contact with the opposing broad side walls of the centralwaveguide section 16, heat generated therein by the microwave will betransmitted to these side walls and immediately dissipated. Thus, thereis no fear of overheating the ferrite element 26 so that it can alwaysfunction as desired. In addition since the intensity of the magneticfield applied to the ferrite element is about several hundred gausses,the exciting elements 27 may be of small size. For instance, they may bedisc shaped permanent magnets having a diameter of to mm. and athickness of 15 mm. Thus, as the construction of the connection 14 issimple and small size it can be manufactured at low cost. In addition tothis advantageous construction, it is very effective to prevent troublescaused by reflected microwaves because of its high transmission loss inthe reverse direction of more than 25 db.

Referring again to FIG. 1, as the H plane T-branch waveguide 15 isarranged with its H plane in parallel with the side wall 112, theoverall height of the heating cavity 10 and the waveguide structure canbe reduced. Ideally, in order to reduce the height it is advantageouslyto intimately contact the H plane T-branch waveguide 15 against theupper wall 112. If the second and third branches 172 and 173intersecting at right angles are mounted on the upper wall .112, thespace required can be saved and the width of the structure can bereduced. As the upper wall 112 is a rectangle having length and width ofseveral ten centimeters, it is very easy to mount the central waveguidesection 16 and the second and third branches 172 and 173 on the upperwall 112. From the standpoints of fabrication and adjustment, it ispreferable to arrange the waveguide branches 172 and 173 in parallelwith the side wall 11. If the end of the first waveguide branch 171opposite to the joint 161 were projected slightly away from wall 112,the magnetron oscillator 13 could be conveniently connected to the lowersurface of the projected portion of the first branch 171. As themicrowave energy is radiated, the magnetron oscillator 13 will be heatedconsiderably, the above described arrangement permits the cooling of themagnetron alone by the independent cooler, not shown, whereby tostabilize its oscillation.

In the embodiment shown in FIG. 1 a circular column shaped ferriteelement 26 is mounted to contact with the opposing inner surfaces ofbroad side walls of the central waveguide section 16. In this case theheight of the ferrite element 26 was made to be equal to the width ofnarrow side walls; or about 55.3 mm. Though a heat loss is generated inthe ferrite element 26 as the microwave is transmitted at the element26, the construction shown in FIG. 1 is satisfactory when the energy ofthe microwave oscillated from the microwave oscillator 13 is at theorder of several hundred watts. However, when the average output of themicrowave oscillator 13 increases to more than 1 kw., the temperature ofthe ferrite member 26 mounted between upper and lower side walls will beincreased due to heat generated therein. Thus, during transmission oflarge microwave energy the temperature of the ferrite element may exceedthe Curie point, which is of course undesirable.

The following embodiment shows a modification of the microwave heatingapparatus of this invention wherein the ferrite element can maintain itssuitable temperature, thus providing adequate isolation effect even whenhandling average output of the microwave energy of more than 1 kw. Thismodification will be described by referring to FIGS. 5 to 7 in whichonly the connector 14 comprising the circulator and adapted tointerconnect them is shown, since the construction and arrangement ofthe cavity and magnetron oscillator are the same as those shown in FIG.1.

The H plane T-branch rectangular waveguide 15 of the H mode is identicalto that shown in FIG. 1. Again, the waveguide includes a firstrectangular waveguide branch 171, a second rectangular waveguide branch172 and a third rectangular waveguide branch 173 which are arranged in aletter T configuration and radially branched at the H plane. Two ferriteelements 41a and 41b are disposed in the central waveguide section 16 ata point slightly displaced from the point of branching 40 or a point ofintersection of the X-X' axis and the Y-Y' axis which divide equally inbroad side of'the branch 171, 172 and in that of 173, respectively. Morespecifically, on the inner walls of broad side walls 27a and 27b of thewaveguide section 16 are mounted a pair of thin disc-shaped ferriteelements 41a and 41b at said point. The ferrite elements comprise dischaving a diameter of 45 mm. and o a thickness of 5 mm., for example,with flat surfaces 42a and 42b and 43a and 43b, for the surfaces 42a and42b contacting with said inner walls 27a and 27b. On the oppositesurfaces 28a and 28b of the broad side walls are secured a pair ofexciting elements 44a and 44b in the form of disc-shaped permanentmagnets which supply cross-field in the direction of an arrow 30 shownin FIG. 6 to ferrite elements 41a and 41b. Thus, the connection 14 actsas a circulator.

Accordingly, the microwave injected into the first branch 171 coupled tothe magnetron oscillator will be transmitted with low loss toward thesecond branch 172, the opposite end thereof being coupled to the heatingcavity. The reflected microwave energy injected into the second branch172 from the heating cavity is transmitted at low loss into the thirdbranch 173 which contains an electric wave absorber 31. Thus theconnection 14 operates as if it were an isolator. The following goodresults were obtained when an object in the heating cavity was heated bythe microwave energy supplied by the magnetron oscillator.

While a pair of thin disc shaped ferrite elements 41a and 41b weresubstituted for an elongated ferrite element 26 shown in FIG. 3, whentransmitting a microwave energy of 2450 met/seen, the insertion loss offerrite in the direction of transmission was about 0.2 db, thetransmission loss in the opposite direction was more than 25 db. It isto be particularly noted that when microwave of said frequency wastransmitted at an average power of 3 kw., the temperature rise offerrite elements 41a and 41b was only 25 C. above room temperature.Thus, even with such a large power, it was possible to maintain thetemperature of the ferrite elements 41a and 41!; at an adequate valuewithout utilizing any cooling device. In-

addition, the isolation of more than 25 db was satisfacory to protectthe magnetron oscillator from the reflected wave. These satisfactoryresults are very desirable to high capacity microwave heating apparatushandling large microwave energy of more than several hundred watts.

Reasons for such good results lie in flat surface of the ferriteelements, correspondingly in tight contact between the ferrite elementsand the side walls 27a and 27b, and lie in extremely thin thickness.These factors contribute to greatly decrease in the ferrite insertionloss in the direction of transmission as well as the heating of theferrite elements caused by the transmission loss of the reflected wavefrom the cavity side. Ferrite elements utilized in the modificationshown in FIG. 5 can be manufactured very readily and economically bymass-production scale because they are in the form of fiat discs. Whilethe configuration of the ferrite elements may be triangular, square orany other shape, circular configuration is preferable because of itseasy fabrication and easiness of determining its center, thusfabricating its positioning on said branching point 40. Thus, althoughthe configuration of the ferrite elements is not critical it ispreferable to make their thickness thin and to make their oppositesurfaces flat and have substantially the same configuration. Further itshould be understood that thickness of the ferrite elements is notlimited to aforementioned value but it was confirmed by experiment thatelements having a thickness thinner than their maximum width can providethe same merits as said example. For example, in circular disc shapedelements having a diameter of 45 mm., thin thickness is selected to beless than 22.5 mm. However they must have a certain thickness becausethey are required to provide a phase difference of about 90 betweenleft-hand and right-hand polarized waves in the waveguide when microwaveis transmitted through ferrite elements. According to experiment, athickness less than about 2 mm. seemed appropriate.

While in the above description regarding the embodiment shown in FIGS. 5to 7, a pair of disc shaped ferrite elements are mounted on the opposinginner walls of broad side walls in order to provide a connector of thecirculator construction one of the ferrite elements may be omitted asshown in FIG. 8. In FIG. 8, branched rectangular waveguide has not beenshown because it is identical to that employed in the above-mentionedembodiments. Thus, as shown in FIG. 8 only one disc shaped ferriteelement is mounted in the central section 16 of the H plane T-branchrectangular waveguide, at a position slightly displaced from the centertoward the third branch 173, as in FIG. 7, said position being acircularly polarized position at which the microwave transmitted throughthe central section 16 of the waveguide creates a circular polarizedmagnetic field. An exciting element in the form of a disc shapedpermanent magnet 51 is mounted on the outer wall 28a to oppose theferrite element 50 whereby to supply a magnetic field distributed asshown by arrows 52 to the ferrite element to cause it to operate as acirculator. However, as an electromagnetic wave absorber 31 is containedin the third branch 173, the connection itself of the circulatorconstruction operates as an isolator to attentuate the microwavereflected from the heating cavity.

Ferrite element 50 mounted on the inner wall 27a is a circular dischaving a diameter of 50 mm. and a thickness of 5 mm. for example, andthe exciting element 51 is a disc shaped barium ferrite permanent magnethaving a diameter of 55 mm. and a thickness of 10 mm. With such a thinferrite element 50 with contact to the inner wall 27a of the broad sidewall, when microwave of a frequency of 2450 mc./sec. transmitted, theinsertion loss of the ferrite in the direction of transmission was only0.2 db while the transmission loss in the opposite direction was morethan 25 db. When microwave energy of the mean power of 1 kw. wassupplied the temperature rise above room temperature of ferrite element50 was only 5 C. Thus, so long as the thickness of the ferrite elementis sufficiently thin, use of only one ferrite element 50 and only oneexciting element 51 is suflicient for practical use. In fact it was ableto eliminate troubles caused by the reflected wave, under light load orno-load condition. Even when the diameter of the ferrite magnet or theexciting element 51 was selected smaller than that of the ferriteelement 50, electrical characteristics, viz the ferrite insertion lossin the direction wave transmission and the transmission loss in theopposite direction did not deteriorate.

In the heating apparatus shown in FIGS. 5 to 7, since a pair of ferriteelements are mounted on the opposed inner walls of the waveguide, it isnecessary to shape them symmetrically to face each other and to mountthem symmetrically to face each other. However, in the modificationshown in FIG. 8, as only one ferrite is utilized such symmetricalshaping and arrangement are unnecessary. Moreover, this arrangement ismore economical because the cost of material of the ferrite element andof the exciting element is reduced to one half. However, in thearrangement shown in FIG. 8, the ferrite element will be subjected totwice heating effect of the configuration of the ferrite element werethe same as that uti lized in FIGS. 5 to 7. As a consequence, in thearrangement shown in FIG. 8 it is desirable to make thin as far aspossible the thickness of the ferrite element. As a result of experimentit was confirmed that the heating effect could be ignored if thethickness of the single ferrite element utilized in the modificationshown in FIG. 8 were less than one half of the maximum width. Forexample, in a disc shaped ferrite element having a diameter of 50 mm.,the thickness may be less than 25 mm.

FIG. 9 shows a still further modification of this invention wherein theH plane branched T-shaped rectangular waveguide comprising theconnector, the heating cavity and the magnetron oscillator are arrangedin a different manner. An H plane T-branch rectangular waveguide 15 ismounted on the upper wall 112 of a heating cavity 10. Again a first, asecond, and a third rectangular waveguide branches 171, 172 and 173 arebranched from a central waveguide section 16 to form a letter Tconfiguration. However, this modification is different from previousembodiments in that the first and the third rectangular waveguidebranches 171 and 173 are disposed on the same straight line and that thesecond rectangular waveguide branch 172 is arranged to intersect saidstraight line at right angles. An antenna 22 which radiates microwaveenergy is projected into the first branch 171 to couple it with amagnetron oscillator 13. The second branch 172 is coupled with theheating cavity through a microwave inlet opening 24. Within the thirdbranch 173 with its outer end closed is positioned an electric waveabsorbing block 31 having a tapered end. Like the arrangement shown inFIG. 6, a pair of circular discshaped ferrite element 41a and 41b aremounted on the opposing broad side walls 27a and 27b of the centralwaveguide section 16, and a pair of exciting elements 42a and 42b aremounted on the outer surface 28a and 28b to oppose ferrite elements 41aand 4115, respectively, whereby to supply a magnetic field to theferrite elements in the direction shown by the arrow 30 in FIG. 10. Withthis construction, the microwave energy supplied to the second branch172 will be bent and transmitted to the heating cavity 10. Whereas themicrowave energy reflected from the heating cavity 10 will be bent anddirected into the third branch 173 to be absorbed and attenuated by themicrowave absorber 31. In this embodiment too, the connection 14intercoupling the magnetron oscillator 13 and the heating cavity 10 isconstructed to operate as a circulator, thus operating stably as anisolator to attenuate the reflected microwave. Losses of the connectionin the forward and reverse directions were substantially the same asthose of afore-described embodiments. It is of course necessary to coolthe magnetron oscillator by means of a suitable cooling device to removeheat generated therein during operation. Utilization of a cooling fan500 results in the following advantages. Fan blades 520 driven by anelectric motor 510 induce a flow of forced cooling air along said firstand third branches arranged on a straight line. In order to eliminate tothe maximum extent troubles caused by the reflected wave, or to attainthe object of this invention, it is necessary to prevent the temperaturerise of the microwave absorbing body 31 caused by the absorbedmicrowave. The cooling fan 500 firstly cools the magnetron oscillatorwhich is liable to suffer overheating, then cools the central waveguidesection 16 to prevent undesirable temperature rise of ferrite elements41a and 41b and finally cools the third rectangular waveguide branch 173to cool the microwave absorbing body 31 contained therein. By constantcooling of the absorbing body 31 it is able to prevent positivelytroubles caused by reflected waves even when the apparatus is usedinadvertently over a long time. Collinear arrangement of the magnetronoscillator 13 requiring cooling and the third branch 173 housing theelectromagnetic wave absorbing body 31 enables effective cooling bymeans of a common cooling fan.

As shown in FIGS. 1 to 9, there have been shown two types of H planeT-branch type rectangular waveguide comprising connections of thecirculator construction and adapted to couple magnetron oscillators toheating cavity. However, another arrangement of the H plane T-branchtype rectangular waveguide comprising the circulator structure may beused wherein the second and the third rectangular waveguide branches arearranged to intersect with the first section at right angles. Of coursesaid second and third branches are arranged collinearly. In thismodified arrangement the first branch is coupled with a magnetronoscillator, the second branch to a heating cavity and the outer end ofthe third branch is closed to contain a microwave absorbing body.

Thus there are provided three different arrangements for the magnetronoscillator, heating cavity and micro wave absorbing body by variousarrangements of the H plane T-branch type rectangular waveguidesdisclosed in the foregoing modifications. Any desired one of thearrangements may be used dependent upon the field of application. Thepossibility of such three different arrange ments can be well understoodfrom FIG. 4 which shows the principle of the novel connection of thecirculator construction. More particularly, if the first, second andthird waveguide branches branched from the central waveguide section inthree different directions were arranged in the forward direction in theorder mentioned, the relative position among respective branches wouldbe independent of the condition required to comprise the circulatorstructure. Thus, although in the above embodiments the circulatorstructures were comprised by H plane T-branch type rectangularwaveguides, the circulator structure can also be provided by an H planebranched Y shaped rectangular waveguide wherein respective Waveguidebranches are branched from the central waveguide section at the H planein the form of a letter Y.

Further more than three rectangular waveguides may be branched indifferent directions from the central waveguide section whereby toconnect the first branch to the magnetron oscillator, the second branchto the heating cavity and to insert the electric wave absorbing body inthe third branch. Remaining branches may be used for other purposes.Alternatively, instead of an H plane branched rectangular waveguide, anE plane branched rectangular waveguide can also be used wherein morethan three rectangular waveguide branches are branched in differentdirections at E plane. However, for microwave heating apparatus producedin mass-production scale, it is advantageous to construct the circulatorby utilizing the H plane branched T-shaped rectangular waveguidesbecause the latter waveguides are easy to fabricate and can be readilycoupled to heating cavity and magnetron oscillator.

As has been discussed hereinabove, troubles caused by reflected wave canbe positively eliminated by intercoupling the heating cavity and themagnetron oscillators by utilizing the novel connection of thecirculator construction which is simple in construction, easy tofabricate and suitable for large power microwave. The ferrite elementand the exciting element utilized for applying magnetic field to theferrite element, the essential elements of the connection comprising thecirculator, are relatively expensive, especially when the excitingelement has the form of a permanent magnet of barium ferrite in whichcase both elements are made of a material of ferrite series which isrelatively inexpensive.

A further modification of this invention shown in FIGS. 11 through 14relates to an improved construction of connection of the circulatorcnostruction wherein the quantity of ferrite material is reduced as faras possible. As shown in FIG. 11, the connection 14 of the circulatorconstruction and adapted to interconnect the heating cavity and themagnetron oscillator 13 comprises an H plane branch T-shaped rectangularwaveguide identical to those illustrated in the foregoing embodiments. Apair of disc shaped ferrite elements 41a and 41b are secured to opposingbroad side walls 27a and 27b of the central waveguide section 16 and apair of exciting elements comprising disc shaped permanent magnets aremounted on the outside of these side walls to oppose ferrite elements.The outer end 60 of the first rectangular waveguide branch 171 oppositeto the branched portion 161 to the central section 16 is closed by ashort circuiting end wall 61. As shown in FIG. 13A, the region betweenthis end and a point spaced therefrom by I for example 160 mm.,constitutes a branch for coupling to the magnetron oscillator, which isdefined by narrow side walls 62a and 62b, each having a width of 55.3mm. and broad side walls 63a and 63b each having a width of 96 mm.Between the branch 65 for coupling to the magnetron and the branchedportion 161 is provided a branch section 66 which includes broad sidewalls of a width of 96 mm. which is the same as the width of said broadside walls 63a and 63b. However the width of the narrow side walls 6701and 67b is smaller than that of the narrow side walls 62a and 62b of thebranch 65 and has a value of 27 mm. Thus, the first rectangularwaveguide branch 171 is comprised by the branch 65 for coupling to themagnetron and said branch section 66, the joint between them beingstepped. An antenna 22 of the oscillator 13 extends into be branchsection 65 perpendicularly through an opening provided through thebottom wallof the branch section. The second branch 172 is disposedcollinearly with the first branch 171 and has the same cross sectionalarea as the first branch. The opposite end of the second branch iscoupled to the heating cavity 10 via wave inlet opening 24.

The third branch 173 is disposed at right angles with respect to firstand second branches 171 and 172 and is designed to have the same crosssectional area as that of the branch section 66. A microwave absorbingbody 31 is contained in the closed end of the third branch 173.

As the H plane branch T-shaped waveguide comprises the first, second andthird branches 171, 172 and 173 of the dimension and relative positionmentioned above, the Width I of the narrow side wall of the centralwaveguide section 16 if 27 mm. In other words, the spacing between broadside walls 27a and 27b of the central waveguide section is equal to 27mm. As can be clearly noted from FIG. 12, the branch section 65 projectsabove the upper surfaces of other waveguide portions including thecentral waveguide section 16 which have the same height. Since thecommonly used standard width of the broad side wall of conventionalrectangular waveguides for transmitting microwave energy of 2450 mc./see. is 96 mm. and that of the narrow side wall is 55.3 mm., only thebranch section of the H plane branched T- shaped rectangular waveguide15 adapted to couple to the magnetron oscillator is required to have thesame dimension as the standard dimension and dimensions of otherportions of the rectangular waveguide, especially of the narrow sidewalls have been reduced to about one half of the standard dimension.Decrease in the width of the narrow Walls results in the followingadvantages. More particularly, as the spacing between opposing excitingelements 42a and 42b decrease, magnetic field applied to the ferriteelements can be increased. Stated in another way, the physical size ofthe exciting elements 42a and 42b can be reduced correspondingly, thusenabling to save ferrite permanent magnets. Further it is able to reducethe volume of the ferrite elements 41a and 41b. For example they can beshaped into discs, 40 mm.-diameter and about 5 mm.-thick, for example,thus greatly decreasing the materials for exciting elements and ferriteelements.

Where a connection of the circulator construction having especiallysmall narrow side walls is utilized to conple the magnetron oscillatorto the heating cavity, the heating cavity containing the object to beheated, as viewed from the magnetron oscillator side, always satifiesthe condition of matched load. Provision of irises 67a and 67b atpositions illustrated by dotted lines in FIG. 11 further improvesmatching. As the first rectangular waveguide section 171 is joined in astep like fashion with the branch section 65 adapted to couple to themagnetron oscillator and the branch section 66 having different width ofnarrow side walls, according to the conventional arrangement it would beimpossible to transmit at high transmission efficiencies microwaveenergy from the magnetron oscillator to the central waveguide section 16unless providing a stepped matching means which is A) in lengths (Xrepresents the wavelength in the waveguide of the microwave having afrequency of 2450 mc./ sec.). However, such a matching means of /4)\ inlengths is not necessary because the spacing between the antenna 22 ofthe magnetron oscillator and the short circuiting end wall 61 isdetermined by the conditions to be described later.

According to various experiments made by the applicants, best matchingcondition does not always results maximum output, but instead, it wasfound that in order to obtain maximum output it is necessary to add areactance of a certain value. The value of this reactance can beadjusted by varying the spacing l between the short circuiting end wall61 behind the antenna and the antenna 22. FIG. 14 shows measured valuesof the microwave energy in the heating cavity 10 supplied from themagnetron oscillator by variously changing said spacing l by moving theshort circuiting end wall 61 while maintaining the spacing 1 between theantenna 22 and the stepped end 68 of the branch section 65 at a constantvalue, 40 mm. for example. The abscissa of FIG. 14 represents thespacing between the antenna 22 and the short circuiting end wall 61while the ordinate the relative output power of the magnetron oscillatormeasured in the heating cavity 10. Where a magnetron 2M66 having themean output of 800 w. is utilized in the magnetron oscillator 13, themaximum output was obtained when the short circuiting end wall 61 waspositioned at distances of 25 mm. and 102 mm. from the antenna as shownby a curve A. Whereas when a magnetron 2M89 having the mean output of1.4 kw. is utilized, the maximum output could be obtained when the shortcircuiting end wall 61 was positioned at distances of 40 mm. and mm.from 13 the antenna as shown by a curve B. Further when a magnetronM4514B having the mean output of 2.5 kw. is used the maximum output wasobtained when the short circuiting end wall 61 was positioned at adistance of 97 mm. from the antenna 22, as shown by a curve C.

Thus, it can be noted from FIG. 14 that maximum output can be obtainedwhen the distance between the antenna 22 and the short circuiting endwall 61 is selected in ranges of 11 mm. to 54 mm. and 86 mm. to 128 mm.respectively. Accordingly even though the height of the centralwaveguide section containing the ferrite element is smaller than that ofthe branch section adapted to couple to the magnetron oscillator themaximum microwave energy will be supplied to the heating cavity byselecting the distance between an antenna and the short circuiting endwall of said branch section. It was found that the connection acting ascirculator as shown in FIG. 11 operated as a satisfactory isolator toprevent troubles ensued by the reflected waves, and that the insertionloss in the direction of transmission was less than 0.2 db and theattenuation in the opposite direction was more than 25 db.

As is obvious from the foregoing descriptions regard ing variousembodiments of this invention, the microwave heating apparatus of thisinvention is especially suitable to operate as a so-called electronicrange because it utilizes a simple connector of the circulatorconstruction which can positively prevent troubles caused by reflectedmicrowave. Further in spite of addition of such a preventing means, thephysical dimension of the connection does not become bulky. The novelmicrowave heating apparatus can also be used for dielectric heating andworking of various materials and articles.

While various embodiments of the invention have been described in theforegoing specification and illustrated in the accompanying drawings,the invention is not to be considered as limited thereto. The scope ofthe invention is defined solely in the appended claims.

What is claimed is:

1. A microwave heating apparatus comprising a microwave oscillator forsupplying microwave energy; a metal wall structure defining asubstantially closed heating cavity; and a rectangular waveguideconnection between said oscillator and said cavity, said connectionincluding:

at least first, second and third branched rectangular waveguidesinterconnected by a central waveguide section having a rectangular crosssection, a first rectangular waveguide branch being coupled to saidoscillator to receive microwave energy therefrom, and a secondrectangular waveguide branch being coupled to said cavity, said first,second and third branches extending radially in different directionsfrom said central section;

at least one ferrite element mounted on the inner side walls of saidcentral waveguide section at a circularly polarized position, saidferrite element being in the form of a plate having a flat surface incontact with said inner side walls;

means including an exciting element mounted on said central waveguidesection adjacent said ferrite element to apply thereto a magnetic fieldsuch that said microwave energy is introduced into said heating cavitythrough said central waveguide section and said first and secondbranches, and to direct microwave energy reflected from said cavity intosaid third branch; and

an electromagnetic wave absorber contained in said third branch toabsorb said reflected energy.

2. The microwave heating apparatus according to claim 1 wherein saidconnection comprises:

at least first, second and third H plane branched rectangular waveguidesinterconnected by a central waveguide section having a rectangular crosssection, a first rectangular waveguide branch being coupled to saidoscillator to receive microwave energy therefrom, and a secondrectangular waveguide branch being coupled to said cavity, said first,second and third branches being radially branched from said centralwaveguide section at the H plane; at least one ferrite element which maybe secured on at least one of the opposing broad side walls of saidcentral waveguide section at a circularly polarized position, saidferrite element being in the form of a plate having a flat surfacecontacting said side wall; means including an exciting element mountedon the outer side of said at least one broad side wall of said waveguidesection adjacent said ferrite element to apply magnetic field to saidferrite element to transmit said microwave energy to said heating cavitythrough said central waveguide section and said first and secondbranches, and to direct microwave energy reflected from said heatingcavity into said third branch; and microwave energy absorber containedin said third branch. 3. The microwave heating apparatus according toclaim 2 wherein said exciting element is a permanent magnet.

4. The microwave heating apparatus according to claim 1 wherein saidconnection comprises:

at least first, second and third H plane branched rectangular waveguidesinterconnected by a central waveguide section, having a rectangularcross section a first rectangular waveguide branch being coupled to saidoscillator to receive microwave therefrom, and a second rectangularwaveguide branch being coupled to said heating cavity, said first,second and third waveguide branches being radially branched from saidcentral waveguide section at the H plane; a pair of ferrite elementssecured to the opposing inner surfaces of the broad side walls of saidcentral waveguide section at a circularly polarized position, saidferrite elements being in the form of plates having flat surfacescontacting said inner surface; means including a pair of permanentmagnets mounted on the outer surfaces of said broad side walls to opposesaid ferrite elements to apply a magnetic field across said centralwaveguide section to transmit the microwave energy in the forwarddirection into said heating cavity through said central waveguidesection and through said first and second branches, and to directmicrowave energy reflected from said heating cavity toward said thirdbranch; and a microwave absorber provided in a said third branch toabsorb said reflected wave. 5. The microwave heating apparatus accordingto claim 1 wherein said connection comprises:

at least first, second and third H plane branched rectangular waveguidesinterconnected by a central waveguide section having a rectangular crosssection, said first rectangular waveguide branch being coupled to saidoscillator to receive microwave energy therefrom, and said secondrectangular waveguide branch being coupled to said heating cavity, saidfirst, second and third waveguide branches being radially branched fromsaid central waveguide section at the H plane; at least one circulardisc shaped ferrite element mounted on the inner surface of at least oneof the opposing broad side walls of said central waveguide section at acircularly polarized position, said ferrite element means having flatsurface contacting said inner surface; means including an excitingelement in the form of a disc shaped permanent magnet mounted on theouter surface of one of said broad side walls to oppose said ferriteelement to apply thereto a magnetic field across said central waveguidesection to transmit the microwave energy in the forward direction intosaid heating cavity through said central waveguide section and throughsaid first and second branches, and to direct microwave reflected fromsaid heating cavity toward said third branch; and

a microwave absorber provided in said third branch.

6. The microwave heating apparatus according to claim 1 wherein saidconnection comprises an H plane branched T-shaped rectangular waveguideinterconnected by a central waveguide section having a rectangular crosssection, a first rectangular waveguide branch being coupled to saidoscillator to receive microwave therefrom, a second rectangularwaveguide branch being coupled to said heating cavity and a thirdrectangular waveguide branch, said first, second and third waveguidebranches being branched from said central waveguide section at the Hplane in three directions to form a letter T configuration a pair ofcircular disc shaped ferrite elements mounted on the inner surfaces ofopposing broad side walls of said central waveguide section at acircularly polarized position, said ferrite elements having flatsurfaces contacting said inner surfaces, means including exciting meansin the form of a pair of disc shaped permanent magnets which are mountedon the outer surfaces of said broad side walls to respectively opposesaid ferrite elements to apply thereto a magnetic field across saidcentral waveguide section whereby to transmit the microwave in theforward direction into said heating cavity through said centralwaveguide section and through said first and second branches and todirect microwave reflected from said heating cavity toward said thirdbranch, and a microwave absorber provided in said third branch.

7. The microwave heating apparatus according to claim 5 whereinsaidbranched rectangular waveguide comprises an H plane branched T-shapedrectangular waveguide interconnected by a central waveguide sectionhaving rectangular cross section, a first, a second and a third branchesbeing branched from said central section at the H plane in threedirections to form a letter T configuration.

8. A microwave heating apparatus comprising a microwave magnetronoscillator for supplying microwave energy, a metal wall structuredefining a substantially closed heating cavity, one of the metal sidewalls defining said cavity being substantially fiat and being providedwith an inlet opening for admitting said microwave, and a waveguideconnection between said oscillator and said heating cavity, saidconnection including an H plane branched T-shape waveguideinterconnected by a central waveguide section having a rectangular crosssection, a first rectangular waveguide branch on said one metal sidewall of cavity, said first branch being coupled to said oscillator atone end thereof opposite to its other end joined to said centralwaveguide section to receive microwave from said oscillator, a secondrectangular waveguide branch collinear with said first branch, saidsecond branch being coupled to said microwave inlet opening at one endthereof opposite to its joint to said central section, a

and a third rectangular waveguide section perpendicular to said firstbranch, said first, second and third branches being branched from saidcentral waveguide section at the H plane parallel to said side wall inthree directions to form T-shaped configuration, at least one circulardisc shaped ferrite element mounted on the inner surface of opposingbroad side walls of said central waveguide section at a circularlypolarized position, said ferrite element having a flat outer surfacecontacting said inner surface, means including an exciting element inthe form of a disc shaped permanent magnet which is mounted on the outersurface of one of said broad side walls to oppose said ferrite elementto apply thereto a magnetic field across said central waveguide sectionwhereby to transmit the microwave in the forward direction into saidheating cavity through said central waveguide section and through saidfirst and second branches and to direct microwave reflected from saidheating cavity toward said third branch and a microwave absorberprovided in said third branch.

9. A microwave heating apparatus comprising a microwave magnetronoscillator for supplying microwave energy, a metal wall structuredefining a substantially closed heating cavity, one of the metal sidewalls defining said cavity being fiat and provided-with an inlet openingfor admitting microwave, and a microwave connection between saidoscillator and said heating cavity, said connection including an H planebranched T-shaped rectangular waveguide interconnected by a centralwaveguide section having a rectangular cross section, a firstrectangular waveguide branch on said one metal side wall, said firstbranch being coupled to said oscillator at one end thereof opposite toits other end joined to said central waveguide section to receivemicrowave from said oscillator, a second rectangular waveguide branchperpendicular to said first branch, said second branch being coupled tosaid microwave inlet opening at one end thereof opposite to its joint tosaid central section, and a third rectangular waveguide branch collinearwith said first branch, the outer end of said third branch opposite toits joint to said central section being closed, said first, second andthird branches being branched from said central waveguide section at theH plane parallel to said side wall in three directions to form T-shapedconfiguration, at least one circular disc shaped ferrite element mountedon the inner surface of opposite broad side walls of said centralwaveguide section at a circularly polarized position, said ferriteelement having a flat outer surface contacting said inner surface, meansincluding an exciting element in the form of a disc shaped permanentmagnet which is mounted on the outer surface of one of said broad sidewalls to oppose said ferrite element to supply thereto a magnetic fieldacross said central waveguide section whereby to transmit the microwavein the forward direction into said heating cavity through said centralwaveguide section and through said first and second branches and todirect microwave reflected from said heating cavity toward said thirdbranch and a microwave absorber provided in said third branch.

10. A microwave heating apparatus comprising a magnetron oscillator forsupplying microwave energy, said oscillator having rod shaped antennasurrounded by a protective cover transmitted to microwave and adapted toextract and radiate generated microwave, a metal wall structure defininga substantially closed heating cavity and a waveguide connectionincluding an H plane branched T-shaped waveguide interconnected by acentral waveguide section having a rectangular cross section; a firstrectangular waveguide branch, one end of said first branch on the sideopposite to its other end joined to said central waveguide section beingclosed by a short circuiting end wall, a predetermined length betweensaid closed end and said junction having relatively narrow width to forma branch section for coupling to said magnetron oscillator, remainingportion between said joint and said branch portion and contiguousthereto constituting another branch portion having the same width as thenarrow side wall of said central waveguide section but shorter than saidnarrow side wall, said antenna of said oscillator extending into saidbranch section for coupling to said magnetron oscillator perpendicularlythrough an opening provided through one of said broad side walls of saidwaveguide, a second rectangular waveguide branch collinear with saidfirst branch and coupled to said heating cavity on the side opposite toits junction to said central waveguide section, said second branchhaving substantially the same cross section as said branch section, anda third rectangular waveguide branch perpendicular to said first branchand having the same cross section as said branch section, the end ofsaid third branch on the side opposite to its branched portion to saidcentral waveguide section being closed, said first, second and thirdwaveguide branches being branched from said central waveguide section atthe H plane in these direc tions to form a letter T configuration, atleast one circular disc shaped ferrite element mounted on the inner sunface of a pair of opposing broad side walls of said central waveguidesection at a circularly polarized position, said ferrite element havinga fiat surface engaging said inner surface, means including an excitingelement in the form of a disc shaped permanent magnet which is mountedon the outer surface of one of said broad side walls to oppose saidferrite element to supply thereto a magnetic field across said centralwaveguide section whereby to transmit the microwave in the forwarddirection into said heating cavity through said central waveguidesection and through said first and second waveguide branches and todirect microwave reflected from said heating cavity toward said thirdbranch and a microwave absorber provided in said third branch.

11. The microwave heating apparatus according to claim 10 wherein saidantenna is provided at a position spaced from said short circuiting endwall by 11 mm. to 54 mm.

12. The microwave heating apparatus according to claim 10 wherein saidantenna is provided at a position 18 spaced from said short circuitingend wall by 86 mm. to 128 mm.

13. The microwave heating apparatus according to claim 1 wherein saidmicrowave absorber is tapered toward said central waveguide section.

References Cited UNITED STATES PATENTS 3,015,787 1/1962 Allin et a1.333-242 10 3,089,101 5/1963 Chait et a1. 33.3-24.2 3,210,513 10/1965Lenart 219-1055 3,311,849 3/1967 Bosma 333-24.1 3,319,191 5/1967 Dixon333-1969 15 RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

U.S. Cl. X.R. 20 333-242

